Crystal phase effect of iron oxides on the aerobic oxidative coupling of alcohols and amines under mild conditions: A combined experimental and theoretical study

Journal of Catalysis 377 (2019) 145–152

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Crystal phase effect of iron oxides on the aerobic oxidative coupling of alcohols and amines under mild conditions: A combined experimental and theoretical study Longlong Geng a,c,1, Wei Jian b,1, Pei Jing a, Wenxiang Zhang a, Wenfu Yan a, Fu-Quan Bai b,⇑, Gang Liu a,⇑ a b c

State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, China Institute of Theoretical Chemistry, Jilin University, Changchun 130012, China College of Chemistry and Chemical Engineering, Dezhou University, Dezhou 253023, China

a r t i c l e

i n f o

Article history: Received 18 November 2018 Revised 31 May 2019 Accepted 11 June 2019

Keywords: Iron oxides Imine synthesis Oxidative coupling reaction Crystal phase effect Molecular oxygen activation

a b s t r a c t Selective catalytic oxidation using air as the terminal oxidant is an ecofriendly route for the synthesis of fine and commodity chemicals. However, the catalyst generally faces the challenges of the inertness of molecular oxygen, limited substrate scope, poor selectivity, and high cost. Moreover, the toxicity of the catalyst should also be considered when the products are used in pharmaceutical or biotechnological areas. Here, upon investigating the dependence of catalytic oxidation on the crystal phases of iron oxides, we find that naked c-Fe2O3 particles exhibit excellent catalytic activity, selectivity, and stability in a series of imine synthetic reactions. The performance of c-Fe2O3 particles is significantly better than that of a-Fe2O3 and Fe3O4 under mild reaction conditions, and the c-Fe2O3 catalyst can be separated from the reaction mixture magnetically. Both experimental and theoretical calculation results show that cFe2O3 possesses supercapability for oxygen activation. The inverse spinel structure of c-Fe2O3 has abundant cation vacancies, which confers unique electronic properties on surface Fe species. These Fe species 2
tend to transfer electrons to molecular oxygen to form O
2 or O2 species. These oxygen species are favorable for the dehydrogenation of alcohols, which is responsible for the high activity of c-Fe2O3 in this coupling reaction. Ó 2019 Elsevier Inc. All rights reserved.

1. Introduction Iron is an attractive metal element for catalysis not only because of its abundant reserves in the earth’s crust, but also because of its low toxicity to organisms and its environmentally benign effects [1–6]. As one of the essential elements of the human body, iron is mainly found in hemoglobin in the form of ions and plays the role of transporting oxygen [7,8]. However, the activation and transport capability of oxygen for most iron oxides is not ideal, because it usually requires relatively high temperature and pressure [9–12]. This limitation restricts the catalytic application of iron oxides in aerobic oxidation under mild conditions (<100 °C, 1 atm), especially the synthesis of some thermally unstable pharmaceutical intermediates. Imines are nitrogen compounds containing the reactive C@N moiety, which are intermediates widely used in biological, agricul-

⇑ Corresponding authors. 1

E-mail addresses: [email protected] (F.-Q. Bai), [email protected] (G. Liu). Longlong Geng and Wei Jian contributed equally to this work.

https://doi.org/10.1016/j.jcat.2019.06.018 0021-9517/Ó 2019 Elsevier Inc. All rights reserved.

tural, and pharmaceutical synthesis [13–20]. Oxidative coupling of alcohols and amines under mild reaction conditions is currently being intensively researched for imine synthesis [21–26], with air and water as the oxygen source and sole by-product. This reaction proceeds in two consecutive steps: oxidative dehydrogenation of an alcohol to an aldehyde and coupling of the aldehyde to an imine with an amine [25,26]. One of the most important requirements for the catalyst is to avoid polymerization and hydrogenation of the reactive C@N moiety [27–31]. Some catalysts with strong dehydrogenation ability, such as supported noble metal catalysts [32–35], also have strong hydrogenation ability, resulting in lower selectivity of the target products. Transition metal oxide catalysts have some advantages in improving the selectivity of imine synthesis [36–40]. However, because of toxicity concerns and separation problems, many toxic heavy-metal-mediated catalytic processes are not suitable for pharmaceutical or biotechnological applications [39]. Although iron-based oxides are favorable catalysts, current research shows that iron oxides (mainly a-Fe2O3) exhibit very low activity in this reaction, mainly due to poor O2 activation ability under mild reaction conditions [20].

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Chemisorption and dissociation are the fundamental steps in molecular O2 activation, which are essentially associated with charge transfer from the active sites of the catalyst to the antibonding molecular orbitals of O2. The crystal phase determines the geometric and electronic structure of a catalyst [41–45], which significantly affects the bulk and surface charge transport, and ultimately reflects the activity and density of the active sites. In this work, by investigation of the dependence of catalytic oxidation on the crystal phases of iron oxides, we find that naked c-Fe2O3 particles (with a magnetically separable character) exhibit excellent catalytic activity, selectivity, and stability for a series of imine synthesis reactions under mild conditions (even at 40 °C, 1 atm). Through a combination of experimental characterizations and theoretical calculations based on density functional theory (DFT), the crystal structure effect of iron oxides on O2 activation and catalytic oxidative coupling behavior is clarified. It is shown that the inverse spinel structure of c-Fe2O3 (with abundant cation vacancies) confers unique electronic properties on surface Fe species. These Fe species tend to transfer electrons to molecular oxygen to form 2
O
2 or O2 species, which is favorable for high activity of c-Fe2O3 in the aerobic coupling reaction. 2. Experimental 2.1. Preparation of iron oxide with different crystal phases Fe3O4 was synthesized via a modified ammonia-assisted precipitation method [46]. Typically, Fe chloride precursors (Fe2+/Fe3+ molar ratio = 1:2) were dissolved in an ethanol/water (1:1) solution under nitrogen with constant mechanical stirring for 30 min, and a 10% ammonia solution was used to adjust the pH of the solution to 9.0. The black suspension was then stirred for 1 h at room temperature and another 1 h at 60 °C in an oil bath. After the system was cooled to room temperature, the resulting solids were separated using a magnet, washed with alcohol, and dried at 80 °C in a vacuum oven. c-Fe2O3 was obtained via thermal treatment of the synthesized Fe3O4 in air at 350 °C for 2 h. a-Fe2O3 particles were obtained using a procedure similar to that for Fe3O4 nanoparticles. The difference is that iron chloride hexahydrate (FeCl36H2O) was used as an Fe precursor and the resulting solids were finally thermally treated at 550 °C for 2 h in a muffle furnace. 2.2. Characterization of catalysts Scanning electron microscopy (SEM) images were taken on a Hitachi-SU8020 with an accelerating voltage of 30 kV. The crystal structures were recorded using a Rigaku powder X-ray diffractometer (XRD) equipped with a CuKa radiation source ( k = 1.542 nm). Raman spectra were collected on a Bruker RFS 100 Raman spectrometer with an argon laser (532 nm) as an excitation source. Fourier transform infrared spectra (FTIR) were recorded on a Thermo Nicolet 6700 FT-IR spectrometer. 57Fe Mössbauer spectra were recorded by an OIMS-500 Mössbauer spectrum instrument. 57 Co (Pd) was used as a c-ray radioactive source and the velocity was calibrated against standard a-iron foil. N2 adsorption– desorption isotherms were measured at 77 K, using a Micromeritics ASAP 2010 N analyzer. Samples were degassed at 423 K for 15 h before measurements. Magnetic measurements were performed on a vibrating sample magnetometer (YingPu VSM-400 Instruments) at room temperature under an applied field of 40,000 Oe.

flask. The flask was connected to an air balloon and the mixture was vigorously stirred at 80 °C for 8 h. Aliquots of the products were taken with a sampling pipe and analyzed with a gas chromatograph equipped with a HP-5 column and FID detector. The product mixtures were further identified by GC-MS. A hot filtration test was carried out as follows: After 2 h of reaction, the solid catalysts were separated with a Buchner funnel. Then the mixture of the filtrate was put into the reactor and continuously reacted under the same conditions (80 °C, air 1 atm) without any solid catalyst. In the recycling experiment, the used catalyst was separated from the reactant by an external magnet and treated at 350 °C for 1 h in air before the next test cycle. 2.4. Density functional theory calculation details All first-principles calculations in this study were performed within the frame of DFT using the plane-wave pseudopotential approach and implemented by the Vienna Ab Initio Simulation Package (VASP). A spin-polarized exchange–correlation functional was described within the PBE generalized gradient approximation [47,48], and electron–core interactions were used with projected augmented wave (PAW) pseudopotentials. The reciprocal space was spanned with a plane-wave basis cutoff of 400 eV. The Brillouin zone was automatically generated by the original Monk horst–Pack grid, and the k-points for structure optimization were selected as 3  3  1. In all cases of structural optimization, the energy convergence was set to 1  10
4 eV, and the force on the atoms was less than 0.05 eV/Å. Fe3d74s1 and O2s22p4 valence electron configuration were adopted in this study. To more accurately consider the effect of the strong correlation of Fe on the 3d electrons, a DFT+U strategy was used to calculate the electron density. This method also helped to correct the O2 binding errors [49,50]. In addition, according to the widely approved values in the literature [51-53], the effective U-values of Fe and O atoms were set to 4.0 and 0.0 eV, respectively. Integration over the irreducible part of the Brillouin zone was carried out using the linear tetrahedron method with Bloch corrections, which depict electronic structure more precisely. Considering the Bader analysis, charges in adsorption are described as numbers of electrons gained or lost in comparison with the neutral atom [54,55]. The lattice parameters of a-Fe2O3, Fe3O4, and c-Fe2O3 used in all calculations are summarized in Table S1 in the Supporting Information. In view of surface and molecular oxygen activation properties, the {0 0 1}, {1 1 1}, and {1 1 1} facets of a-Fe2O3, Fe3O4, and c-Fe2O3 were adopted, respectively [56–58]. To eliminate the influence of the thickness of the slab atomic layers, all models were composed of eight layers of Fe and four layers of O and separated from periodic images by at least 15 Å of vacuum. Concerning the significance of the adsorption interface, the top five layers were relaxed and the following seven layers of atoms were fixed in the optimization process. The adsorption energy can be expressed as

DEads ¼ Esurf þO2
Esurf
EO2 where Esurf þO2 ,Esurf , and EO2 represent the total energy of the adsorbed oxygen molecules, the energy of the surface, and the gas-phase energy of oxygen molecules, respectively. 3. Results and discussion 3.1. Preparation and characterization of iron oxide catalysts

2.3. Catalytic test A mixture of benzyl alcohol (1 mmol), aniline (2 mmol), catalyst (0.3 g), and toluene (10 mL) was added into a 50-mL two-neck

Considering potential applications, three readily available and stable iron oxides, hematite (a-Fe2O3), magnetite (Fe3O4), and maghemite (c-Fe2O3), were synthesized and systematically

L. Geng et al. / Journal of Catalysis 377 (2019) 145–152

147

Scheme 1. Illustration of crystal structures of (A) a-Fe2O3, (B) Fe3O4, and (C) c-Fe2O3. Color scheme: pink, oxygen; purple, iron.

investigated. Both crystal structures and the oxidation state of iron are taken into account in the selection process. Scheme 1 shows the crystal structures of a-Fe2O3, Fe3O4, and c-Fe2O3. a-Fe2O3 has a corundum-type structure with Fe3+ ions occupying two-thirds of the octahedral sites in a hexagonal close-packed O lattice. Fe3O4 features an inverse spinel crystal structure of oxide anions arranged as a cubic close-packed lattice, containing atoms of Fe (II) and Fe(III) in octahedral and tetrahedral sites. c-Fe2O3 possesses a cubic structure consisting of 32 O2
ions, 21 1/3 Fe3+ ions, and 2 2/3 vacancies in each unit cell. Restricted by the cubic close-packed O lattice arrangement, ferric ions distribute over tetrahedral and octahedral sites which are remaining Fe ions and vacancies [59]. The test samples of a-Fe2O3 and Fe3O4 were prepared by a modified precipitation method in which ethanol and ammonia were used as solvent and precipitant, respectively, to control the nucleation and hydrolysis rate of iron ions. FeCl3 and a stoichiometric mixture of FeCl3 and FeCl2 are used as starting reagents for the preparation of a-Fe2O3 and Fe3O4, respectively. Both of the resultant iron oxide particles occur together with smaller particles of size 22–35 nm (Fig. 1A and 1B, SEM images). A large number of mesopores (about 15–23 nm) form the packing of these small particles (Fig. S1 and Table S2, N2 adsorption–desorption results). c-Fe2O3 was obtained by thermal treatment above Fe3O4 precursor in air. The transformation of Fe3O4 to c-Fe2O3 is a solid-state oxidation process [60,61], with no obvious morphology change being observed (Fig. 1C, SEM images). The crystal structures of three iron oxides are first confirmed by X-ray powder diffraction (XRD) (Fig. 1D). The rhombohedral structure of a-Fe2O3 and the cubic structure of Fe3O4 and c-Fe2O3 can be identified [62]. Taking into account that c-Fe2O3 has the same XRD patterns as Fe3O4, FTIR, Raman, and 57Fe Mössbauer measurements are further carried out to confirm the c-Fe2O3 phase (Fig. 1E–1G). Combined with the above characterization results and corresponding fitting analysis (Table S3), this finally allows us to verify the purity of the obtained catalyst [63,64]. 3.2. Catalytic performance of iron oxides in imine synthesis The catalytic oxidative coupling behavior of these iron oxides was first investigated in the imine synthesis reaction with benzyl alcohol and aniline as reagents and air as oxygen source. The imine is identified using gas chromatography–mass spectrometry. c-Fe2O3 exhibits much higher activity for this reaction than a-Fe2O3 and Fe3O4 (Fig. 2A). A 91.3% yield can be obtained over c-Fe2O3 after reaction for 8 h at 80 °C. The selectivity to imine is higher than 99% and almost no by-products except water are detected. This result clearly shows that c-Fe2O3 could effectively

avoid polymerization and hydrogenation of the reactive C@N moiety. Under the same reaction conditions, c-Fe2O3 also exhibits obvious active advantages over a series of reducible and irreducible oxide catalysts (Table 1). c-Fe2O3 can catalyze this reaction at very low temperatures, yielding 68.5% and 39.1% imines at 60 and 40 °C, respectively (Table 1, entries 5 and 6). Under a 1/1molar ratio of benzyl alcohol/aniline, the yield of imine is 61.6% (Table 1, entry 7), indicating that a relatively high concentration of aniline could be suitable for optimizing the reaction process. Calculated from the initial reaction rate, the apparent activation energy (Ea) of the reaction over c-Fe2O3 is about 59 kJ mol
1, which is obviously lower than that of Fe3O4 (77 kJ mol
1) and a-Fe2O3 (102 kJ mol
1, Fig. S2). All these results show that c-Fe2O3 is an efficient catalyst for imine synthesis from oxidative coupling of benzyl alcohol and aniline. Another advantage of c-Fe2O3 is its magnetically separable character. Fig. 2B shows that c-Fe2O3 has very strong superparamagnetic properties. The saturation magnetization (Ms) is 75.5 emu/ g, which is comparable to that of bulk c-Fe2O3 materials (Ms = 76 emu/g) [65,66], while the remnant magnetization (Mr) is only 0.5 emu/g. This property facilitates separation of c-Fe2O3 from the reaction mixture with an external magnet and good redispersion after the external magnetic field is removed (Fig. 2B, inset). Upon magnetic separation, a hot catalyst filtration test is carried out to investigate the heterogeneous nature of c-Fe2O3. Fig. 2C shows that no subsequent yield of imines is detected in the filtrate, proving that c-Fe2O3 is a heterogeneous catalyst. The stability of c-Fe2O3 is investigated by examining its recyclability in the reaction. As displayed in Fig. 2D, after a test of six cycles, the catalyst still maintains an activity as high as the fresh one. The SEM image (Fig. S3) and XRD pattern (Fig. S4) show that used c-Fe2O3 still maintains the morphology and cubic structure of fresh c-Fe2O3. No crystal transformation from the c-phase to the a-phase occurs in the reaction process. These results show that c-Fe2O3 is a stable heterogeneous catalyst for this oxidative coupling reaction. Additionally, the general application of c-Fe2O3 catalyst is investigated. At least ten imines are achieved by the oxidative coupling of different kinds of alcohols and amines. Table 2 shows that c-Fe2O3 catalyst is active in the reactions using benzyl alcohol derivatives with either electron-rich or electron-poor substituent groups, and excellent imine yields are obtained. c-Fe2O3 is also active in catalyzing oxidative coupling of benzyl alcohol with substituted anilines and obtains high imine yields. Aliphatic amines, such as cyclohexane and n-butylamine, can also react with benzyl alcohol to afford the desired imines in good yields by prolonging the reaction time. These results show that the c-Fe2O3 catalyst has high universality for imine synthesis with an oxidative coupling route.

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Fig. 1. SEM images of (A) a-Fe2O3, (B) Fe3O4, (C) c-Fe2O3; (D) XRD patterns, (E) FT-IR spectra, and (F) Raman spectra of different iron oxide catalysts and (G) 57Fe Mössbauer spectra of c-Fe2O3 particles.

We also demonstrate that the reaction catalyzed by c-Fe2O3 is an aerobic process. As displayed in Fig. 3A, only a small amount of imine is detected in relatively inert environments. Benzaldehyde is an intermediate product of the reaction, which is produced by oxidative dehydrogenation of benzyl alcohol (Fig. 3B). The benzyl alcohol dehydrogenation rate for these three samples follows the order c-Fe2O3 > Fe3O4 > a-Fe2O3 (Fig. S5). The Ea of benzyl alcohol dehydrogenation is 51 kJmol
1 on c-Fe2O3 and 78kJmol
1 on Fe3O4 (Fig. S6). All these trends are consistent with oxidative coupling of alcohol and amine. The following step in the coupling of benzaldehyde and aniline is a fast reaction, either on c-Fe2O3 catalyst or on a-Fe2O3, and Fe3O4 catalysts (Fig. 3C and Fig. S7). Therefore, it can be concluded that oxidative dehydrogenation of alcohols is the rate-determining step for imine synthesis. 3.3. O2 activation mechanism over different iron oxides DFT calculations were carried out to understand the O2 activation mechanism over different iron oxides. Based on the surface activity, the facets of {0 0 1}, {1 1 1}, and {1 1 1} are considered as models for a-Fe2O3, Fe3O4, and c-Fe2O3, respectively [56– 58,67,68]. Calculated surface energy results show that c-Fe2O3 is

the most sensitive to oxygen among the three, followed by Fe3O4 (Fig. S8). Fig. 4 shows the optimized configurations of O2 adsorption on the active facets of a-Fe2O3, Fe3O4, and c-Fe2O3. The corresponding bond length and adsorption energy (Eabs) are listed in Table S4. It shows that adsorbed oxygen molecules mainly form FeAO4 bonds on the surface of a-Fe2O3 (Fig. 4A). Two different FeAO bond lengths, 1.81 Å and 2.56 Å, are obtained (Table S4), which indicates that weak interaction is present between Fe species and O5. On the surfaces of Fe3O4 and c-Fe2O3, oxygen molecules are more favorable to forming identical FeAO bonds. The bond length is 1.84 Å for Fe3O4 and 1.81 Å for c-Fe2O3, respectively (Table S4). The Eabs results show that the adsorption of oxygen molecules on three iron oxides is all the exothermic processes, and Eabs follows the order c-Fe2O3 (
4.3 eV) > Fe3O4 (
1.84 eV) > a-Fe2O3 (
1.58 eV). Additionally, the OAO bond lengths of adsorbed oxygen are also quite different on the surfaces of iron oxide catalysts. They are 1.29, 1.39, and 1.41 Å for a-Fe2O3, Fe3O4, and c-Fe2O3, respectively. The OAO distance of adsorbed oxygen on c-Fe2O3 is obviously 2
greater than that of O
2 (1.33 Å) and approximately that of O2 (1.49 Å) and H2O2 (1.48 Å) [69]. All these results show that the cFe2O3 surface is very active for oxygen activation, followed by that

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(A)

100

(B)

γ-Fe2O3

80

40 60

M (emu/g)

Imine yield (%)

80

Fe3O4 40

Mr=0.5 emu/g Ms=75.5 emu/g Hc=6.7 Oe

0

-40

α-Fe2O3

20

-80

0 0

2

4

6

8

-40000

-20000

Reaction time (h) 100

(D)

Imine yield (%)

80

Filterition After

Before

60

20000

40000

100 80

Imine yield (%)

(C)

0

Magnetic Field (Oe)

40 20

60 40 20

0 0

1

2

3

4

5

6

0

1

2

3

4

5

6

Recycle times

Reaction time (h)

Fig. 2. (A) Time dependence of imine formation from oxidative coupling of benzyl alcohol and aniline over iron oxide catalysts. (B) Magnetic hysteresis loop of the synthesized c-Fe2O3 catalyst. (C) Leaching of c-Fe2O3 by continuing the reaction after filtration of the catalyst (the red line indicates the imine yield after the catalysts are removed). (d) Reusability of c-Fe2O3 in the imine synthesis process. Reaction conditions: benzyl alcohol 1 mmol, aniline 2 mmol, catalyst 0.3 g, toluene 10 mL, 80 °C, air 1 bar.

Table 1 Imine synthesis from benzyl alcohol and aniline over various metal oxides.a

Entry 1 2 3 4 5 6 7 8 9 10 11 12 a b c d e f

Catalyst –

a-Fe2O3 Fe3O4

c-Fe2O3 c-Fe2O3 c-Fe2O3 c-Fe2O3 NiO Co3O4 CeO2 Al2O3 La2O3

Yield [%]b c

<1 13.5 52.5 91.3 68.5d 39.1e 61.6f 29.9 23.6 17.5 3.1 1.4

Entry

Catalyst

Yield [%]b

13 14 15 16 17 18 19 20 21 22 23

MgO ZrO2 Cr2O3 CuO TiO2 Bio Cano MoO3 WO3 Sb2O3 ZnO

1.3 1.0 <1 <1 <1 <1 <1 <1 <1 <1 <1

Reaction condition: benzyl alcohol 1 mmol, aniline 2 mmol, metal oxide 0.3 g, toluene 10 mL, 80 °C, air 1 bar. Isolated imine yield. Reaction time 12 h. Reaction temperature 60 °C. Reaction temperature 40 °C. Benzyl alcohol 1 mmol, aniline 1 mmol.

of Fe3O4. Moreover, adsorbed oxygen on the c-Fe2O3 surface possesses much greater oxidative capability. Fig. 5 shows the partial density of states (PDOS) of different iron oxides before and after adsorption of oxygen. The valence bands of iron oxides are composed of Fe3d and O2p orbits, and the conduction bands consist of Fe3d orbits (Fig. 5A–C). The calculated band gaps of a-Fe2O3, Fe3O4, and c-Fe2O3 are 1.16, 1.25, and 1.81 eV, respectively. The Fermi level of a-Fe2O3 and c-Fe2O3 is near the top of the valence band, and the Fermi level of Fe3O4 is close to

the bottom of the conduction band. After adsorption of oxygen, the band gap becomes narrower and the Fermi level moves to lower energy (Fig. 5D–F). The Fe3d orbit exhibits a relatively low strength and overlap with the p-orbital of the adsorbed O, indicating that Fe is bonded to the oxygen molecule. O2p orbits of three iron oxides change after oxygen molecule adsorption, indicating that charges also transfer between Fe and crystal oxygen during the adsorption process. For O2p orbits of adsorbed oxygen, three spikes can be observed clearly on c-Fe2O3 in the range from
2

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Table 2 Oxidative coupling of alcohols and amines to imines over c-Fe2O3 catalyst.a Reaction time (h)

Yield (%)b

1

8

91.9

2

8

98.9

3

8

93.2

4

8

86.5

5

8

81.2

6

8

95.2

7

12

69.1

8

8

92.3

9

12

85.4

10

12

78.9

Product

a

Reaction conditions: alcohol 1 mmol, amine 2 mmol, catalyst 0.3 g, toluene 10 mL, 80 °C, air 1 atm. b The yield of imines was determined using GC-MS.

to
7 eV, which is different from those of Fe3O4 and a-Fe2O3. This indicates that c-Fe2O3 possesses a greater capacity for molecular oxygen activation.

(C) 100

80

80

80

Conversion & yield (%)

(B) 100

Imine yield (%)

(A) 100

60 40 20

Imine yield (%)

Entry

Bader charge analysis is further used to determine the charge distribution of Fe and O atoms around adsorption sites of different iron oxides (Table 3). For a pristine model slab, the effective charges of the Fe atom in a-Fe2O3, Fe3O4, and c-Fe2O3 are 1.63, 1.52, and 1.21, respectively. The effective charge of O in c-Fe2O3 is about
1.20, which is greater than that of Fe3O4 (
0.96) and a-Fe2O3 (
0.94). This result shows that the FeAO bond in cFe2O3 has a greater contribution from covalent bonding than those in a-Fe2O3 and Fe3O4. This indicates that more electrons are shared between Fe and O atoms in c-Fe2O3. After adsorption of oxygen, a significant change can be observed from the Bader charges of Fe and O in c-Fe2O3. This indicates that it is much easier for c-Fe2O3 to transfer electrons to adsorbed oxygen. This trend is followed by Fe3O4, while little change can be found in a-Fe2O3. The electronic properties of different iron oxides further confirm that c-Fe2O3 possesses the best ability for oxygen activation. According to our experimental results and corresponding literature [24–26], oxidative dehydrogenation of alcohols is the ratedetermining step for imine synthesis. The subsequent coupling step of aldehydes with amines occurs easily on three iron oxides (Fig. 3C and Fig. S7). Tsunami et al. have reported that super oxoor peroxo-like species could abstract a b-hydrogen atom from the alcohol to generate the corresponding aldehydes or ketones [70]. Therefore, the O2 activation capability of catalysts is the key factor for the whole catalytic process. In our case, the theoretical calculation results identify that c-Fe2O3 possesses favorable properties for charge transfer between Fe and O species, which can activate 2
molecular O2 to form O
species. These oxygen species 2 or O2 are favorable for the dehydrogenation of alcohols, which is responsible for the higher activity of c-Fe2O3 than of Fe3O4 and a-Fe2O3. The O2 activation capability should be essentially ascribed to the special inverse spinel structure of c-Fe2O3, which has abundant

60 40 Benzyl alcohol conversion Imine yield

20

Benzaldehyde yield

0

0 Air

N2

Reaction atmosphere

0

2

4

6

Reaction time (h)

8

γ-Fe2O3

60 40 20

Catalyst free

0 0

60

120

Reaction time (min)

Fig. 3. (A) Effects of reaction conditions (air or N2) on catalytic performance of c-Fe2O3 catalyst in imine synthesis. (B) Time dependence of imine synthesis by oxidative coupling of benzyl alcohol and aniline over c-Fe2O3 catalyst. (C) Time dependence of imine formation by coupling of benzaldehyde and aniline with and without c-Fe2O3 catalyst. Reaction conditions: benzyl alcohol or benzaldehyde 1 mmol, aniline 2 mmol, catalyst 0.3 g, toluene 10 mL, 80 °C, air or N2 1 atm.

Fig. 4. Optimized configurations of O2 adsorption on the facets of (A) a-Fe2O3 {0 0 1}, (B) Fe3O4 {1 1 1}, and (C) c-Fe2O3 {1 1 1}. The purple and pink colors represent Fe and O atoms, respectively. O4 and O5 represent adsorbed oxygen molecules. For clarity, the symmetrical portion of the bottom slab is not shown.

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Fig. 5. Partial density of states (PDOS) for (A) a-Fe2O3 {0 0 1} clean surface, (B) Fe3O4 {1 1 1} clean surface, (C) c-Fe2O3 {1 1 1} clean surface, (D) O2 adsorption on a-Fe2O3 {0 0 1} surface, (E) O2 adsorption on Fe3O4 {1 1 1} surface, and (F) O2 adsorption on c-Fe2O3 {1 1 1} surface. Fe is an iron atom in the surface adsorption site and O represents oxygen atoms around the surface adsorption site (O3, O4, and O5). Dashed vertical lines mark the position of the Fermi level, and the Fermi level is set to 0 eV.

Table 3 Bader charges on Fe, O, and adsorbed O atoms in different iron oxides. Sample

Fe

O1

O2

O3

O4

O5

a-Fe2O3–slab a-Fe2O3–O2

1.63 1.65 1.52 1.65 1.21 1.62


0.94
0.97
0.96
0.94
1.20
1.12


0.94
0.96
0.96
0.94
1.19
1.17


0.95
0.93
0.97
0.95
1.21
1.16

–
0.24 –
0.38 –
0.37

–
0.12 –
0.28 –
0.38

Fe3O4–slab Fe3O4–O2 c-Fe2O3–slab c-Fe2O3–O2

cation vacancies. This structure determines the electronic properties of the surface Fe and O atoms of c-Fe2O3. 4. Conclusions Based on crystal phase regulation and screening, we obtain an active and robust iron oxide catalyst for oxidative coupling in imine synthesis. c-Fe2O3 not only possesses high activity and selectivity under mild conditions, but also manifests the recycling characteristics of magnetic recovery. Suitability for activating molecular oxygen is the critical factor for c-Fe2O3 in oxidative dehydrogenation of alcohols. Theoretical analysis of surface, geometric structure, and electronic structure shows that c-Fe2O3 possesses excellent capability for molecular oxygen activation. Compared with a-Fe2O3 and Fe3O4, the surface Fe atoms of c-Fe2O3 easily transfer electrons to molecular oxygen to form O
2 or O2
species. This work provides molecular-level insights into 2 the crystal phase effect on catalytic performance in molecular oxygen activation, and can shed light on developing other metal oxide catalysts for molecular-oxygen-activation-related reactions. Acknowledgments The authors acknowledge support from the National Natural Science Foundation of China (21473073), the Development Project of Science and Technology of Jilin Province (20170101171JC,

20180201068SF) and Open Project of the State Key Laboratory of Inorganic Synthesis and Preparative Chemistry (201703), and the Fundamental Research Funds for the Central Universities. L. Geng thanks Dr. Haixiang Han from Cornell University for the discussion and improvement of this article and the Natural Science Foundation of Shandong Province for its support (ZR2018LB018).

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